Expose implementation?

[patricoferris]

Currently, the implementation of a runtime type value ('a t) is abstract to any library that uses repr. From the perspective of repr this is nice because it allows the authors to freely change the implementation with hopefully very little downstream impact and most users are probably happy with the set of features provided by repr.

However, for anybody wishing to extend repr this is not possible as they cannot look at the implementation to manipulate it. I've hit this a few times. First when prototyping an automatic diffing library (I ended up prototyping with typegist instead https://github.com/patricoferris/patch) and more recently when thinking of doing some SQL generation for types. Perhaps I'm missing something in the API that would let me extend repr in a separate library? If not, would there be interest in providing this feature? Even something like:

val to_type_core : 'a t -> 'a Type_core.t
val of_type_core : 'a Type_core.t -> 'a t

Thanks! :))